In many cases of deformity, such as spondylolisthesis, it is desirable to reduce the extent of displacement of a vertebra prior to fusion to an adjacent vertebra. A spondylolisthesis reduction can be a technically demanding procedure requiring great care to prevent neurological impairment and damage to surrounding soft tissue. As a result, several systems have been utilized to accomplish a necessary reduction.
An early system utilized a cable system to apply force to the displaced vertebrae. One end of an extremely long cable was anchored to a vertebra and the other end was threaded through a fixture attached to the ceiling and included a series of traction weights positioned on the cable. Such a system utilizing long cables and weights external to the operating room proved unwieldy and bulky. Moreover, the surgeon could not quickly and effectively control the force applied to the vertebra.
In addition to cable systems, systems utilizing threaded shafts to draw the vertebra upwards were also used. One such apparatus for use in straightening a spinal column by reduction is presented in U.S. Pat. No. 4,611,581. The apparatus disclosed in this patent includes a pair of rigid plates positioned along the spinal column with a double threaded screw anchored in the vertebrae requiring alignment with the vertebral column. The lower portion of the screw has a cancellous thread, which engages the bone, while the upper portion has a machine thread, which passes through a slot in the plate and is capable of receiving an internally threaded nut. Upon rotation of the internally threaded nut bearing against the plate, the misaligned vertebra is drawn toward the plate as the nut advances over the machine threads on the screw. This system requires that the machine-threaded end of the screw extend through a relatively narrow slot in the plate. Such a requirement may limit the surgeon's ability to properly place the screw or may require manipulation of the vertebrae to align the screw with the slot in the plate.
Other systems have utilized a separate threaded shaft to draw the vertebrae into alignment. In such systems, the bone screw does not include a machine-threaded portion. Instead, a separate reduction mechanism grasps the head of the screw and is braced against a rod. A threaded shaft attached to the screw head pulls the misaligned vertebra toward the rod. Here again, the devices are cumbersome and difficult to maneuver into the appropriate position so the vertebra is pulled in the desired direction. Moreover, the surgeon cannot quickly and effectively control the force applied to the vertebra because he's never completely sure how much tension is being applied to the misaligned vertebra
What is needed is reduction device that utilizes a rigid shaft to pull the vertebra with a device that indicates the amount of pulling force that the surgeon is exerting on the vertebra. The following is one solution to that need.